Grout repair on granite entry

[highup]

Small area......... maybe a foot in total has the old grout chipping out. Entry has been in for 8 to 10 years it I had to guess.
Grout gap is less that 1/8" What's the best way to grout this? Is it possible to do it in one shot........... meaning grout and wipe in (almost) one felled swoop.
The granite tile is black 1 foot squares with some blue flecks in it, and the grout is black.

I was thinking to mask and tape off the granite to within 1/8 of an inch on each side of the grout line then pack the voids with grout and trowel off the excess and then maybe do one pass with a sponge before removing the tape.
...then maybe another light wipe after removing the tape.
Wait for 15 or 20 minutes and do another wipe with the sponge.

I have a bubble repair to do on the carpeted area in another room, so I could do a final wipe before I leave. I figured masking the area to make less mess since the area is so small. No need to make a large mess on some shiny granite tile.
.....obviously Hi isn't a tile guy, but he might be one if it's hourly work. No, it's a friend/client..... just helpin out while I'm there.
Also.......... should I use water or additive to mix the concoction. Most likely it will be Latacrete.... unsanded of course.

 

[Ernesto]

No real need to tape it off unless getting hourly which prolly wouldnt take but twenty minutes anyway.

 

[Csason]

I think you need to get all the old grout out if you are going
to regrout, and then wipe as normal...as you grout.

 

[highup]

I think you need to get all the old grout out if you are going
to regrout, and then wipe as normal...as you grout.

Small area......... maybe a foot in total

It's a tiny area. I'll remove anything that's loose.
The store stocks the Laticrete Permacolor Select grout system. I was thinking just mixing the base and colorant powder for a long time in a 5 gallon bucket to be sure it's mixed completely, then just using a cup of the mixed product for the repair. Not sure that I like the idea of this base/colorant system. What a waste to mix up an entire 12.5lb bag to do 14 inches long, 3/16 wide grout line then toss the rest.
...not to mention $31
Customer showed me another spot 18" long area that had loose grout between the bathroom and shower.............. the bathroom floor and the shower are level to each other. It's a handicap accessible bathroom.
The grout is light in that area........... another $31 for a bag of grout and colorant for this second tiny repair.
Polyblend has those regular small boxes of grout at the local True Value store, but no black and very few color choices. Those would be ideal, and only $14 per box and premixed color ready to go.
Be interesting to see how this Laticrete Select behaves. Says it's fast setting, Kevlar reinforced and can be walked on in a couple hours.
Anything special about using this product?

 

[highup]

Reading the instructions on the Laticrete website, you drop the colorant bag into the water and the bag dissolves................ Doing a color test patch would cost $31 since you have to use the entire gag to do a test.
Can that dye bag be split in half successfully using gram scales and a couple of disposable drink cups?
I have some postal scales to divide up the bag of grout base.